A stainless steel material is excellent in corrosion resistance and also favorable in strength, workability and joint characteristics. Therefore, a stainless steel pipe formed with the stainless steel material is used in various applications due to excellent corrosion resistance and high strength.
Further, in recent years, since reduction in cost has been requested, high-strength stainless steel pipes to be used are required for improving the corrosion resistance and strength without containing expensive elements.
Then, on the basis of percent by mass, a 12% Cr martensite-based stainless steel material is primarily used with the content of Cr increased and the contents of C and N decreased, further Cr, Ni, Mo and Cu are contained in proper amounts to provide a composition. Then, a diploid phase structure is formed with a ferrite phase and a residual austenite phase, with the martensite phase given as a base phase, thereby providing a stainless steel pipe which is improved in strength, hot workability, corrosion resistance and weldability (refer to Patent Document 1, for example).
Further, on the basis of percent by mass, with the content of N decreased to 0.015% or less, a martensite-based stainless steel material is welded to form a stainless steel pipe. Thereafter, the pipe is austenized at 920° C. to 1100° C., cooled at a cooling velocity greater than water cooling, subjected to tempering, and cooled at a cooling velocity greater than air cooling, thereby providing a high-strength stainless steel pipe in which martensite is formed. This high-strength stainless steel pipe is sufficient in corrosion resistance even in a carbon dioxide atmosphere and also excellent in impact toughness and weldability (refer to Patent Document 2, for example).
Still further, with regard to the metallic structure, a proper amount of ferrite phase is introduced into a parent phase of austenite to give a two phase structure based on austenite which contains 5 to 40% by volume of ferrite phase, thereby providing a stainless steel pipe with improved workability and corrosion resistance (refer to Patent Document 3, for example).
A stainless steel pipe in which Mo and V are combined with a ferrite-based stainless steel material in proper contents to improve corrosion resistance, and hot rolling conditions and cold rolling conditions are specified to suppress reduction in workability due to the content of Mo is also available (refer to Patent Document 4, for example).